Currently, mode scramblers for stabilizing outgoing light from a light source has been used in an insertion loss test of an optical fiber and an optical measuring instrument. Among these mode scramblers, there is a mode scrambler having a structure using a graded index (GI) fiber, described in Japanese Utility Model Application Laid-Open No. 61-013803 (hereinafter referred to as Patent Literature 1), as a basic mode scrambler. In this structure, microbending is generated by pressurizing a fiber core wire so as to enable attenuation of a higher-order mode distribution. Further, in Japanese Patent Application Laid-Open No. 2005-099274 (hereinafter referred to as Patent Literature 2), a small and inexpensive mode scrambler is provided by using an optical fiber wiring board in which a multimode optical fiber is laid on a circuit.